Mobile devices, typified by cellular phones, use a semiconductor memory such as a DRAM, SRAM, or flash memory. A DRAM provides large capacity but its access speed is low. A SRAM, on the other hand, is high-speed memory, but is not suitable for forming a large capacity memory, since each cell requires a number of transistors (4 to 6 transistors) and hence it is difficult to produce highly integrated SRAM. DRAM and SRAM must continuously receive power to retain data; that is, they are volatile memories. Flash memory, on the other hand, is a nonvolatile memory; it does not need to continuously receive power to electrically retain data. However, the flash memory is disadvantageous in that its program/erase count is limited to a maximum of approximately 105 and its reprogramming speed is a few orders of magnitude lower than those of other memories. Since each memory (described above) has its disadvantage, it is current practice to select suitable memory depending on the application.
If a universal memory having all the advantages of DRAM, SRAM, and flash memory were developed, a plurality of memories could be integrated on a single chip, which allows cellular phones and other mobile devices to be miniaturized and enhanced in functionality. If the universal memory could replace all other types of memory, it would have a tremendous impact (on the semiconductor industry). The requirements for universal memory are that: (1) like DRAM, it is highly integrated (and hence can have large capacity); (2) its access (write/read) speed is high, comparable to that of SRAM; (3) it has the same nonvolatility as flash memory; and (4) it exhibits low power consumption and hence can be powered by a small battery.
Among next-generation nonvolatile memories referred to as universal memories, phase change memory is currently attracting the most attention. Phase change memory uses a chalcogenide material, which is also used by CD-RWs and DVDs. Like these disks, phase change memory stores data by assuming two states: a crystalline state and an amorphous state. However, they differ in how data is written to or read from them. Specifically, whereas a laser is used to write to or read from CD-RWs and DVDs, the Joule heat generated by an electrical current is used to write data to the phase change memory and the change in the resistance of the memory due to the phase change is read as a data value.
The principle of operation of phase change memory will be described with reference to FIG. 2. When a chalcogenide material is amorphized, such a reset pulse is applied that causes the chalcogenide material to be rapidly quenched after it is heated to a melting point or more. The melting point is, for example, 600° C., and the quench time (t1) is, for example, 2 nsec. When crystallizing the chalcogenide material, on the other hand, a set pulse is applied to the memory so as to maintain the chalcogenide material at a temperature between its crystallization point and melting point. The crystallization point is, for example, 400° C., and the time (t2) required for the crystallization is, for example, 50 nsec. A feature of phase change memory is that the resistance value of the chalcogenide material (of the phase change memory) varies by two to three orders of magnitude depending on its crystallization state. Since (the change in) the resistance value is used as a signal, the read signal is large, facilitating the sense operation and hence increasing the speed of the read operation. Another feature of the phase change memory is that it can be reprogrammed 1012 times, which is an advantage over flash memory. Still another feature of the phase change memory is that it can operate at a low voltage and low power, which allows it to be formed on the same chip as logic circuitry. Therefore, phase change memory is suitable for use in mobile devices.
An exemplary manufacturing process for a phase change memory cell will now be briefly described with reference to FIGS. 3 to 5. First, a select transistor is formed on a semiconductor substrate by a known manufacturing method (not shown). The select transistor is made up of a MOS transistor or bipolar transistor. Then, an interlayer insulating film 1 made up of a silicon oxide film is deposited and a plug 2 of, for example, tungsten is formed in the interlayer insulating film 1 by a known manufacturing method. This plug is used to electrically connect between the select transistor and the phase change material layer overlying the select transistor. Then, a chalcogenide material layer 3 of, for example, GeSbTe, an upper electrode 4 of, for example, tungsten, and a hard mask 5 made up of, for example, a silicon oxide film are sequentially deposited, forming the structure shown in FIG. 3.
Then, the hard mask 5, the upper electrode 4, and the chalcogenide material layer 3 are processed by a known lithographic technique and dry etching technique, as shown in FIG. 4.
After that, an interlayer insulating film 6 is deposited, as shown in FIG. 5.
Then, a wiring layer electrically connected to the upper electrode 4 is formed on the interlayer insulating film 6, and a plurality of other wiring layers are formed on the wiring layer on the interlayer insulating film 6, completing formation of phase change memory (not shown).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-174144    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-229537